The ever increasing world population and subsequent growing demand for agricultural products requires progressively more efficient and improved plant breeding and crop protection methods. Crop pests, such as insects, result in significant reduction of yield and current pest control is reliant on the application of insecticides; however, the development of resistance to insecticides and their often negative impact on human health and the environment demands the development of appropriate and alternative compounds to those currently in use.
Semiochemicals are a class of substances which mediate intra- and interspecific communication between the same or different species. The process by which these signals are recognised is termed olfaction and is the process by which the olfactory signal or ligand is recognised by olfactory recognition proteins, resulting in a particular response. These olfactory recognition proteins are highly specific and capable of distinguishing even structurally related compounds. Semiochemicals are key recognition cues in perfumes and cosmetics or food and beverages, and have a role in the control of pests, particularly insects and are therefore highly sought after compounds. However, semiochemicals are usually extremely volatile, unstable compounds and their chemical synthesis is hugely expensive.
Terpenes and terpenoids are a large and diverse group of organic compounds commonly found in plants ranging from essential primary molecules to more complex secondary metabolites and are known for their semiochemical properties. Pesticide compounds and formulations that include terpenoids are disclosed in WO2013/191758. Terpenes and terpenoids are hydrocarbons assembled of isoprene subunits providing the carbon skeleton which then undergoes further modification. The early core steps in the terpenoid biosynthesis are well characterised utilising the primary building blocks isopentenyl diphosphate (IDP) and dimethylallyl diphosphate (DMADP) and leading to the synthesis of the terpenoid precursors geranyl diphosphate (GDP), farnesyl diphosphate (FDP) and geranylgeranyl diphosphate (GGDP). Terpenes are classified sequentially dependent on their number of isoprene subunits as hemiterpenes (one isoprene subunit), monoterpenes (two isoprene subunits), sesquiterpenes (three isoprene subunits), etc.
Although terpenes and terpenoids are an attractive target for synthetic modification and the modulation of their natural properties may lead to new medicinal and agrochemical compounds with improved and altered functions. The complexity of the hydrocarbon skeletons and the chemical instability of many terpenoids, particular those with semiochemicals properties, can present a difficult challenge to the synthetic chemist. Synthetic biology approaches have focused on the preparation of natural terpenoids utilising whole biochemical pathways in living organisms, or increasing endogenous terpenoid production in plants to repel or attract insects or other organism such as disclosed in US2014/0173771; however, possible substrates for enzymes involved in the terpenoid synthesis are limited in cells and does not result in the generation of modified terpenoids with altered properties.
Germacrene D is known to repel insects, particularly aphids such as the grain aphid (Sitobion avenae) and there is therefore interest in producing analogues of germacrene D which may have modified properties.
Cascón et al, in Chem. Commun., 48, 9702-9704 (2012), have described the synthesis of various germacrene D analogues using fluorine and methyl modified FDPs as a substrate for (S)-germacrene D synthase (GDS). In particular, they synthesised 6-F, 14-F, 15-F and 14-methyl analogues.
However, these germacrene D analogues proved to have reduced activity compared with germacrene D.